The present invention relates to an ultra-fine pulverizing method for obtaining ultra fine particles having a size of several microns or less, which are needed in a high strength concrete, a high performance catalyst or the like.
A recent technology is disclosed in Japanese Pat. Examined Publication No. Hei 5-87307 entitled "Centrifugal Processing Method and Apparatus". The concept of that technology is a vertical mill, as shown in FIG. 15, in which an agitating shaft 02 is provided within a hollow rotor 01 and pulverizing media 03 are disposed in a gap S between the shaft 02 and the rotor 01. Then, under the condition that the material M to be processed is present in the gap S, the hollow rotor 01 is rotated and at the same time, the agitating shaft 02 is rotated in the opposite direction to that of the rotor 01, thereby pulverizing the material M to be processed. According to the publication, the rotational speed is adjusted so that an acceleration exceeding 1 G is applied to the pulverizing media 03, and it is preferable to select the rotational speed in the range of 10 G to 200 G.
Also, according to the publication, it is preferable that when an inner radius of the hollow rotor 01 is represented by R, the above-described gap S meets the relation, 0.50.ltoreq.S/R.ltoreq.0.95, more preferably S/R=0.80 to 0.95. Namely, in the case where the gap S is small (S/R&lt;0.50), the centrifugal force is made-uniform and the pulverizing effect is made uniform but the processing performance degrades. On the other hand, in the case where S/R&gt;0.95, the agitating effect attained by the agitating shaft 02 degrades.
Conventionally, there is a theory prerequisite that "it is preferable to use a high rotational speed and small size pulverizing media". Therefore, the conditional theory suggests high speed rotation of 10 G to 200 G as mentioned above and pulverizing media having a small diameter of 3 mm or less.
However, this high rotational speed and small diameter media type mill suffers from the following problems.
(1) Frictional wear of the pulverizing media is large.
Since the frictional wear rate of the pulverizing media is in proportion to a rotational speed of the mill and a specific surface area of the pulverizing media, the more the acceleration and the smaller the pulverizing media, the more the frictional wear rate will increase as shown in FIG. 5.
(2) Damage rate of the pulverizing media is high.
The greater the diameter of the pulverizing media, the greater the pressure yield strength of the pulverizing media will become. Therefore, in case of the small diameter media, the damage rate of the pulverizing media is high.
(3) Power consumption is large and the temperature of the pulverizing material is high.
The mill power is in proportion to the rotational speed and the amount of heat generated in the mill is in proportion to the mill power. Accordingly, in case of the high rotational speed, the temperature of the pulverized material becomes high. In many cases, the elevated temperature is a factor in the degradation of the quality of the pulverized material or the hindrance against the upgrading the performance.